Field of the Invention
The present invention relates to a sheet processing apparatus, a control method thereof, and an image forming apparatus having the sheet processing apparatus, and more particularly, relates to a sheet processing apparatus that performs saddle stitch bookbinding, and a control method thereof, and an image forming apparatus having the sheet processing apparatus.
Description of the Related Art
A sheet processing apparatus that performs saddle stitch bookbinding, generally overlays a plurality of sheets and folds the sheets in the center (center-folding) at a time to bookbind. In an example of the sheet processing apparatus that performs saddle stitch bookbinding, stripes are formed on the sheets in advance, before the center-folding is performed on the sheets, at parts to be creases formed during the center-folding, and the sheets with the stripes formed thereon are center-folded with the sheets overlaid (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2000-272823). The stripes for the center-folding, which are formed on the sheets, make it possible to easily perform the center-folding on not only regular paper, but also thick paper. The stripes further prevent peeling of a toner image printed on a sheet supposed to be a cover sheet at the bookbinding.
However, the formed stripes produce rising parts on the sheets. Therefore, when a plurality of sheets are center-folded and overlaid, if the stripe-formed positions of the sheets vary, the overlaying of the sheets with the stripes formed thereon may cause gaps between the overlaid sheets. Particularly, if the stripes are formed on all the sheets, the gaps between the overlaid sheets are accumulated at the part to be folded of the sheet bundle, which makes the fold part thick. This may reduce the quality of the finish of the saddle stitch bookbinding.